1. Field of the Invention
The present invention relates to a multi-stage compressor, and, more particularly, to the controlled start-up of the compressor stages.
2. Description of the Related Art
Known uses of conventional multi-stage compressors include heat pump, air conditioning, and refrigeration system applications. Often times such compressors include first and second stage compression mechanisms that are mounted at opposite ends of a drive motor. The drive motor is drivingly linked to each of the first and second stage compression mechanisms by a drive shaft. Typically, the drive shaft engages the first and second stage compression mechanisms so that they are out of phase from one another, i.e., at different points in the compression cycle. Multi-stage compressors wherein each of the compression mechanisms are arranged in series are well suited for applications where a high pressure difference in the working fluid is required, such as when using carbon dioxide. For the compressor to provide such relatively large pressure increases, a relatively large motor is also typically required.
In operation of a two-staged compressor, electrical power is supplied to the motor which in turn simultaneously drives the first and second stage compression mechanisms. Refrigerant is compressed in the first stage from a suction pressure to an intermediate pressure. The intermediate pressure refrigerant is then supplied to the second stage compression mechanism and is compressed from the intermediate pressure to a higher, discharge pressure. The discharge pressure refrigerant is then supplied to the associated system or application, e.g., a refrigeration system.
During start-up of a multi-stage compressor driven by a single motor, the initial start-up current, i.e., the inrush current, of the motor may be several times greater than the operating or steady-state current of the motor. This initial spike of current can be damaging to the motor or power supply and thereby reduce the life of the equipment.